Conventionally, electrophotographic copying machines using a contact charging method form images as follows. First, a surface of a photosensitive drum is uniformly charged by bringing a charging roll into contact with the photosensitive drum. Then, an original image is projected onto the surface of the photosensitive drum via an optical system. By discharging the charged electricity at the portion onto which the light is projected, an electrostatic latent image of the original image is formed thereon. Next, toner is uniformly applied to a developing roll. By coating the electrostatic latent image with the toner, a toner image is formed. Then, the toner image is transferred to transfer paper to form a transferred image.
The developing roll includes a shaft made from metal such as SUS, a conductive elastomeric layer (referred to also as a base layer) that surrounds the surface of the shaft, and a functional layer (referred to also as a surface layer) containing a binder resin, a conductive agent and other additives that surrounds the conductive elastomeric layer. The functional layer may be a single layer, or a multilayer.
The functional layer contains carbon black or an ion conductive agent as the conductive agent, with which a conductive property is provided to the functional layer. The functional layer is about 0.1 to 20 μm in thickness, and the binder resin thereof is made of a polyurethane resin, an acrylic resin or NBR. The elastomeric layer is about 1 to 4 mm in thickness, and is made of rubber such as silicone rubber, hydrin rubber and NBR.
If the content of the ion conductive agent is small in the functional layer of the developing roll, the charging fails to make differences in toner concentration at regular pitches, resulting in a problem of non-uniform image density. To be specific, as shown in FIG. 2, a printed image 110 includes higher-density portions 111 and lower-density portions 112, which alternately appear in a rotation direction of a developing roll 101. The concentration differences of the image density are so-called unevenness in a horizontal stripe pattern.
Meanwhile, if the content of the ion conductive agent is larger in the functional layer, the charging property of the functional layer is improved to prevent unevenness in a horizontals tripe pattern from developing. However, the larger ion conductive agent content causes problems that the toner sticks to the layer, that image unevenness and image contamination are increased by bloom of the ion conductive agent, and that a charged-electric quantity under an environment of high humidity is reduced.
A developing roll that is capable of solving the problem caused by the bloom of the ion conductive agent is known (e.g., PTL 1).
The developing roll of PTL 1 includes a surface layer that contains a binder resin, carbon black, an ion conductive agent, which is made of salt containing specific imidazolium cations and specific anions, and polyamide porous particles.